A coating system gaining wide acceptance, particularly in the automotive industry, is one which is known as "color plus clear". In this system the substrate is coated with one or more applications of a pigmented basecoating composition, which is in turn coated with one or more applications of a generally clear topcoating composition.
However, there are several difficulties in employing "color plus clear" coating systems especially as attempts are made to employ coating compositions having high solids contents and also as metallic flake pigments are used to provide a special two tone appearance to the coated substrate as it is viewed from different angles to a direction normal to the surface of the substrate. For example, it is important in a "color plus clear" coating system that the applied basecoat not be attacked by components of the topcoating composition, particularly solvents, at the interface of the two, a phenomenon often referred to as strike-in. Strike-in adversely affects the final appearance properties of the coated product. Strike-in is an especially serious problem when metallic-flake pigments are employed in the basecoating composition. Strike-in, among other things, can destroy the desired metallic-flake orientation in the basecoat.
Additionally, irrespective of the problems associated with strike-in, it is important to prevent sagging during curing of the coating composition after application to a nonhorizontal substrate. Also, especially where metallic-flake pigments are employed, it is important to achieve and maintain proper pigment orientation in the pigmented basecoating composition during the curing or drying operation. Moreover, where a material is incorporated in the topcoating composition to prevent sagging of the topcoating composition during cure, it is particularly desirable that such material not seriously affect the clarity of the cured topcoat, for example, by imparting to the topcoat a cloudy or milky appearance.
One attempt to address some of these problems has been to incorporate in the basecoating composition as part of the organic polymer system present, a proportion of organic, insoluble polymer microparticles as described for example in U.S. Pat. No. 4,220,679 to Backhouse. Another attempt to address at least some of the problems of achieving proper metallic-flake orientation in a high solids basecoat has been to substantially increase the amount of metallic-flake pigment in the basecoating composition as described in U.S. Pat. No. 4,359,504 to Troy.
It has now been found that the incorporation of substantially inorganic microparticles in the basecoating composition permits the basecoating composition to be formulated for example at high solids content and alleviates the problems of strike-in, the problems of achieving excellent metallic-pattern control where metallic-flake pigments are employed, and the problem of sagging of the coating composition on a nonhorizontal substrate during curing or drying. It has also been found that the incorporation of substantially inorganic microparticles, for example based on silica, in the topcoating composition, not only alleviates sagging of the topcoating composition during cure but also does not seriously affect the clarity of the transparent topcoat.